While the primary function of piezoelectric energy harvesters (PEHs) is to produce electricity from various vibration sources, there is a growing interest to use the same device as a sensor to detect various environmental contexts. For example, researchers have recently demonstrated that attaching a PEH to a wearable device can not only generate electricity from human activities, but the generated electrical signal can also be directly used as a signature to characterise the activity, i.e., running, walking, jumping  [1]}, [2]}, [3]}. Similarly, attaching the PEH device under a vibrating bridge not only generates electricity from the bridge vibrations, the generated voltage responses can be used to detect events of interest such as detecting times of vehicle's entry and exist to the bridge [4]}, identifying train passages [5]} or even detecting bridge damages [6]}. A key benefit of using a PEH as a sensor is that it eliminates the powering requirement of a separate dedicated sensor, such as an accelerometer or strain gauge, thus advancing the possibility of realizing completely self-powered wireless Internet of Things sensors [7]}.
